The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for fabricating a thin film transistor using a polysilicon with a double junction structure.
Thin film transistors have been widely employed as the switching device of liquid crystal displays(LCD).
For the high quality pictures and the high performance of liquid crystal display units, the high speed in the switching operation of thin film transistors(TFT) is required.
It is also required to properly meet the characteristics of thin film transistor such as ON current Ion, OFF current I off, threshold voltage V.sub.th, electric field and mobility.
Hereinafter, a conventional method for fabricating a thin film transistor will be described with reference to the drawings.
FIGS. 1a to 1d are sectional views showing a conventional method for manufacturing a thin film transistor.
First, an insulation film 2 and a polysilicon 3 are formed on a transparent insulation substrate 1 in this order, as shown in FIG. 1a.
As shown in FIG. 1b, thereafter, a gate insulation film 5 is grown on the surface of polysilicon 3 and a polysilicon 6 is then deposited on the gate insulation film 5.
Subsequently, an unnecessary portion of the polysilicon 6 and the gate insulation film 5 is selectively removed using a photolithography process and an etching process to form a gate electrode 6a, as shown in FIG. 1c.
Thereafter, n-type ions having a high concentration are implanted into the polysilicon 3 located under both sides of the gate electrode 6a using the gate electrode 6a as a mask to form a source impurity region 4 and a drain impurity region 4a and further to complete a thin film transistor.
In accordance with a conventional thin film transistor fabricated as above mentioned, upon applying a voltage into the gate electrode 6a, a current start flowing according to the voltage difference between the source impurity region 4 and the drain impurity region 4a.
A channel is formed at the boundary of the polysilicon 3 and the gate insulation film 5 located between the source region 4 and the drain region 4a.
However, such conventional thin film transistor uses a polysilicon as the material of channel layer.
A problem may occur due to the characteristic of polysilicon film quality such as defect elements in polysilicon itself, electric fields at junction regions and results related to these. That is, since a leakage current starts flowing between the source impurity region and the drain impurity region although the gate electrode is turned off, there is a disadvantage in which the picture quality becomes inferior, when thin film transistors are used as switching devices of liquid crystal display units.